1. Field of the Invention
The present invention relates to an image processing apparatus which executes processing that normalizes an image, an image processing method, and a program.
2. Description of the Related Art
There is a method which searches for a similar image by extracting and comparing feature amounts, such as colors and edges of images, from both of an image serving as a search query (hereinafter referred to as a query image) and an image stored in a database (hereinafter referred to as a stored image).
Conventionally, when an image having a color or image quality changed by printing or scanning is searched for, or conversely, when an image of the original is searched for using an image having a changed color or image quality, normalization has been executed by referring to pixel values of the entire image. For example, as preprocessing for extracting a feature amount from an image, a maximum value and a minimum value of a luminance component have been corrected, or a dynamic range (ratio of the maximum value to the minimum value) of one of color components of an image has been corrected. With such normalization, even an image having a changed color or image quality can been handled similar to an image having no change.
Japanese Patent Application Laid-Open No. 2007-012024 discusses a method which quantizes continuous luminance from white to black and normalize luminance values such that a pixel of each quantized luminance appears in equal proportion in an image.
Further, Japanese Patent Application Laid-Open No. 09-44665 discusses a method which searches for a similar image using a local feature amount of an image. This method selects feature points from each image using a DOG function, determines feature amounts corresponding to the selected points, and compares these feature amounts with each other.
However, in an image having a color or image quality changed by scanning or the like, a dot noise (isolated dot noise or halftone dot noise) irrelevant to a surrounding pixel may often be generated. A pixel containing such a noise may have an extreme value of luminance. Even if a technique discussed in Japanese Patent Application Laid-Open No. 2007-012024 is used, the image may not excellently be normalized.
However, on the other hand, if normalization is executed after a filter for correcting a dot noise is applied to the entire image, this normalization may affect a feature amount which is extracted after normalization has been executed, and may influence search accuracy and recognition accuracy.
Particularly, as a technique discussed in Japanese Patent Application Laid-Open No. 09-44665, when an image is recognized and searched for with a feature amount at a predetermined feature point (hereinafter referred to as a local feature amount) set as a search query, localization of the local feature amount may be lost.